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DB2 - Problem description

Problem IC75870 Status: Closed

GLOBAL SNAPSHOTS MAY CONSUME A LARGE AMOUNT OF FCM BUFFERS

product:
DB2 FOR LUW / DB2FORLUW / 950 - DB2
Problem description:
In a multipartition environment, a large amount of FCM buffers 
can be consumed by large global snapshots. One example of such 
large global snapshots is, "db2 get 
snapshot for locks on <dbname> global" when there are many 
locks currently being held in the database. 
 
db2pd -fcm hwm output would show a single application 
consuming a lot of buffers
Problem Summary:
**************************************************************** 
* USERS AFFECTED:                                              * 
* DPF environments                                             * 
**************************************************************** 
* PROBLEM DESCRIPTION:                                         * 
* Global snapshots collect monitoring information from all     * 
* database partitions and aggregate the result at the          * 
* coordinating partition. FCM buffers are required to send and * 
* receive the monitoring information. The coordinating         * 
* partition can consume a large number of FCM buffers          * 
* equivalent to the size of the entire snapshot result.  If    * 
* the database manager runs out of FCM buffers then node       * 
* failure is invoked to remedy the situation. This could lead  * 
* to applications receiving -1229 errors or losing connection  * 
* to the database.                                             * 
*                                                              * 
* Note that monitoring tools like db2top issue global          * 
* snapshots.                                                   * 
**************************************************************** 
* RECOMMENDATION:                                              * 
* Upgrade to DB2 v9.5 FP8 or later and enable flow control.    * 
****************************************************************
Local Fix:
The work around of setting DB2_MAX_GLOBAL_SNAPSHOT_SIZE should 
help to prevent a single snapshot from consuming all FCM 
buffers. Multiple concurrent large snapshots would still be a 
problem though.
available fix packs:
DB2 Version 9.5 Fix Pack 8 for Linux, UNIX, and Windows
DB2 Version 9.5 Fix Pack 9 for Linux, UNIX, and Windows
DB2 Version 9.5 Fix Pack 10 for Linux, UNIX, and Windows

Solution
As of DB2 v9.5 fp8, the fix for this APAR can be enabled by 
using the following registry variable setting: 
db2set DB2_SYSTEM_MONITOR_SETTINGS=ENABLE_FLOW_CONTROL:TRUE 
 
This is a dynamic setting which means the instance does not have 
to be restarted to pick up the registry 
variable setting. With flow control enabled, global snapshots 
will not consume a large number of FCM buffers.
Workaround
not known / see Local fix
Timestamps
Date  - problem reported    :
Date  - problem closed      :
Date  - last modified       :
18.04.2011
27.06.2011
27.06.2011
Problem solved at the following versions (IBM BugInfos)
9.5.FP8,
9.5.FP8
Problem solved according to the fixlist(s) of the following version(s)
9.5.0.8 FixList
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